The correction of tires to reduce force variations is called "tire uniformity optimization" or "TUO", and is disclosed in a number of publications including U.S. Pat. Nos. 3,724,137; 3,725,163; 3,849,942; 3,914,907 and 4,047,338. The apparatus of these patents all sense force variations imparted by a tire rotating against a load drum. They also generate electrical signals representing these force variations. In response to those signals, the methods and apparatus of all except the '163 patent move a grinding wheel into and out of cutting engagement with the tread surface of a tire. Because the force variations are caused by variations in the stiffness and geometrical dimensions of the tire about its circumference, the portions of the circumference where the force transmitted between the tire and the load drum is excessively high in relation to other portions may be corrected by removing rubber from the tire tread. This rubber removal reduces the tire's stiffness and tread thickness in these portions and consequently reduces the amount of force transmitted to levels closer to the force transmitted in other areas. In the method of the '163 patent, additional rubbery material is applied to the tread surface in areas of low force transmission instead of being removed in areas where the force is high.
In previous methods and apparatus for correcting force variations that employ rubber grinding or other cutting devices, such as those shown in the '137, '907, '338 and '942 patents referred to above, hydraulic pistons are used to move the grinding wheels into and out of engagement with the tread rubber. The pistons locate the grinding wheels at the desired cutting depths and are moved by pressurized hydraulic fluid. The fluid is controlled by a servo system that is designed to move the pistons and grinding wheels to their correct grinding positions. However, while the pressurized fluid pushes the grinding wheels in one direction, the tire being buffed pushes back on the grinding wheels; so the exact positioning of the pistons and grinding wheels is determined by a balancing of the pressure in the hydraulic system to position the grind wheel. Also, the pistons can be delayed and can oscillate as they travel to their full cutting depth during this hydraulic pressure/tire force balancing. Thus, one problem with using hydraulic pistons to locate the grinding wheels is that the wheels cannot be set at accurately fixed, predetermined cutting depths at the exact locations of the tire circumference where rubber removal is desired.
Another problem with the previous tire uniformity optimization systems is that the positioning of the grinding wheel is made without reference to the actual position of the surface of the tire where the grinding takes place. The run-out of the tire determines where the surface of the tire is at any given location of grinding, and must be taken into consideration in determining where to locate the grinding wheels to remove a desired thickness of rubber. While some previous systems use the run-out measurement to determine how much hydraulic pressure to apply to the pistons, this is for the purpose of reducing the areas of high run-out, a process which is called "tire trueing". However, this is not the same as using the run-out measurement to establish a reference position for the grinding wheels at each circumferential location on the tire, so that the grinding wheels can be moved inwardly from that reference position by the exact distance equal to the depth of the cutting desired at that location.
Still another problem with previous tire uniformity optimization methods and apparatus is that they are carried out at high speeds, usually about 60 revolutions per minute. While for a passenger tire this is equivalent to running at only about 5 miles per hour, that is a relatively fast speed for correcting tires for non-uniformities. Errors caused by grinding too much or too little or in the wrong areas can occur, and the final appearance of the tire can suffer because each circumferential portion being ground moves by the grinding wheel at a relatively fast rate, resulting in fewer passes of the grinding wheel during each pass of the tread portion. Consequently, a rougher grind is produced.